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EP1587049A1 - Verfahren und Vorrichtung zur Verbesserung der Übereinstimmung einer Anzeigetafel mit einem Anzeigestandard vollflächig und für verschiedene Blickwinkel - Google Patents

Verfahren und Vorrichtung zur Verbesserung der Übereinstimmung einer Anzeigetafel mit einem Anzeigestandard vollflächig und für verschiedene Blickwinkel Download PDF

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Publication number
EP1587049A1
EP1587049A1 EP04447098A EP04447098A EP1587049A1 EP 1587049 A1 EP1587049 A1 EP 1587049A1 EP 04447098 A EP04447098 A EP 04447098A EP 04447098 A EP04447098 A EP 04447098A EP 1587049 A1 EP1587049 A1 EP 1587049A1
Authority
EP
European Patent Office
Prior art keywords
display
greyscale
colour
zone
emissive elements
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04447098A
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English (en)
French (fr)
Inventor
Tom Kimpe
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Barco NV
Original Assignee
Barco NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Barco NV filed Critical Barco NV
Priority to EP04447098A priority Critical patent/EP1587049A1/de
Priority to TW094111813A priority patent/TW200540793A/zh
Priority to JP2007507778A priority patent/JP4890441B2/ja
Priority to US11/578,385 priority patent/US8228348B2/en
Priority to KR1020067021179A priority patent/KR101122982B1/ko
Priority to EP05779896.9A priority patent/EP1735767B1/de
Priority to CNB2005800115038A priority patent/CN100504980C/zh
Priority to PCT/EP2005/004151 priority patent/WO2005101355A1/en
Publication of EP1587049A1 publication Critical patent/EP1587049A1/de
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2092—Details of a display terminals using a flat panel, the details relating to the control arrangement of the display terminal and to the interfaces thereto
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/36—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source using liquid crystals
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G5/00—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators
    • G09G5/02—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators characterised by the way in which colour is displayed
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0233—Improving the luminance or brightness uniformity across the screen
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0271—Adjustment of the gradation levels within the range of the gradation scale, e.g. by redistribution or clipping
    • G09G2320/0276—Adjustment of the gradation levels within the range of the gradation scale, e.g. by redistribution or clipping for the purpose of adaptation to the characteristics of a display device, i.e. gamma correction
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/028—Improving the quality of display appearance by changing the viewing angle properties, e.g. widening the viewing angle, adapting the viewing angle to the view direction
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0285—Improving the quality of display appearance using tables for spatial correction of display data
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/0606—Manual adjustment
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/0626—Adjustment of display parameters for control of overall brightness
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/068—Adjustment of display parameters for control of viewing angle adjustment
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/0693—Calibration of display systems
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2360/00—Aspects of the architecture of display systems
    • G09G2360/14—Detecting light within display terminals, e.g. using a single or a plurality of photosensors
    • G09G2360/144—Detecting light within display terminals, e.g. using a single or a plurality of photosensors the light being ambient light

Definitions

  • the present invention relates to systems and methods for electronic display devices, especially fixed format displays. More particularly, the invention relates to systems and methods for electronic display devices complying with enforced display standards, such as for example medical electronic display devices complying with enforced medical display standards like e.g. the DICOM standard.
  • enforced display standards such as for example medical electronic display devices complying with enforced medical display standards like e.g. the DICOM standard.
  • More and more medical displays are used as replacement for traditional film in radiology. Instead of using expensive film a radiologist looks at a digital image on a high-quality (typically greyscale) medical display.
  • An additional advantage of the medical display is that the radiologist is able to perform image-processing operations on the medical image such as contrast enhancement, zoom ... and this makes it easier to diagnose.
  • medical displays require very high quality and quality control as they are very often used for primary diagnosis and therefore life-critical decision taking. A lot of regulations and recommendations exist.
  • One example of such a quality requirement is the "DICOM/NEMA supplement 28 greyscale standard display function". It describes how the greyscales in a digital medical image should be mapped to the output levels of a medical output device such as a display, a film-printer ... in order to maximise the visibility of small details present in the digital image file.
  • Fig. 1 and Fig. 2 are extracts from the document "DICOM/NEMA supplement 28 greyscale standard display function".
  • Fig. 1 shows the principle of changing the global transfer curve of a display system to obtain a standardised display system 102 according to a standardised greyscale standard display function.
  • the input-values 104 referred to as P-values 104
  • P-values 104 are converted by means of a "P-values to DDLs" conversion curve 106 to digital driving values or levels 108, referred to as DDL 108, in such a way that, after a subsequent "DDLs to luminance" conversion, the resulting curve "luminance versus P-values" 114 follows a specific standardised curve.
  • the digital driving levels then are converted by a "DDLs to luminance” conversion curve 110 specific to the display system and thus allow a certain luminance output 112.
  • This standardised luminance output curve is shown in Fig. 2, which is a combination of the "P-values to DDLs" conversion curve 106 and the "DDLs to luminance” curve 110.
  • This curve is based on the human contrast sensitivity as described by the Barten's model. It is to be noted that it is clearly non-linear within the luminance range of medical displays.
  • the greyscale standard display function is defined for the luminance range 0.05 cd/m 2 up to 4000 cd/m 2 .
  • luminance JND represents the index of the just noticeable differences, referred to as luminance JND, and the vertical axis shows the corresponding luminance values.
  • a luminance JND represents the smallest variation in luminance value that can be perceived at a specific luminance level.
  • a display system that is perfectly calibrated based on the DICOM greyscale standard display function will translate its P-values 104 into luminance values (cd/m 2 ) 112 that are located on the greyscale standard display function (GSDF) and there will be an equal distance in luminance JND-indices between the individual luminance values 112 corresponding with P-values 104.
  • This means that the display system will be perceptually linear: equal differences in P-values 104 will result in the same level of perceptibility at all digital driving-levels 108.
  • the calibration will not be perfect because, typically, only a discrete number of output luminance values (for instance 1024 specific greyscales) are available on the display system.
  • a "DICOM-calibration" with medical display systems which often - but not necessary - are LCD displays, is achieved as it has always been done with CRT-displays: by measuring the native transfer curve of the display, i.e. determining the luminance versus DDL, and using this curve to calculate a conversion table between P-values and DDLs. Measuring the native transfer curve of the display is done by placing a luminance measurement device with small acceptance angle in the centre of the display. A device with small acceptance angle is used because otherwise the variation of viewing angle characteristics of the display make the measurement data unreliable. With a device with a large acceptance angle, the measurement results are integrated values over a wide range of viewing angles.
  • FIG. 5 shows an example of the distortion in percent from the mean luminance value over the complete display area for a fixed viewing angle. Also this luminance uniformity problem over the display area causes very bad DICOM-conformance. For people skilled in the art it will be obvious that especially at the darker video levels, even small luminance variations introduce a large distortion from the ideal DICOM-model.
  • US-5359342 furthermore describes a way to obtain a linear transfer curve for different regions in the display, without normalising the total brightness. Nevertheless, the system does not describe a method for obtaining an optimum DICOM conformance behaviour, whereby the transfer curve is adjusted to the individual variations of display pixels or zones. Furthermore, the correction provided in US-5359342 is a constant correction, not taking into consideration the environmental changes or the conditions in which the display is used.
  • the invention relates to a method for correcting non-conformance in greyscale or colour values of at least a plurality of zones of emissive elements in a matrix display, the correction being with respect to an enforced greyscale or colour display standard, each zone of emissive elements being corrected by a different calibration function.
  • the method comprises for each zone of emissive elements independently, storing characterisation data characterising the non-conformance in greyscale or colour values of the zone of emissive elements as a function of its drive signals and pre-correcting, in accordance with the characterisation data, the drive signals of the zone of emissive elements so as to obtain a greyscale or colour level conform the enforced greyscale or colour display standard, wherein the pre-correcting is performed based on an input value of the greyscale or colour value to be displayed and the viewing angle under which the zone of emissive elements is or is to be viewed at.
  • the zone of emissive elements may consist of one emissive element or the zone of emissive elements may comprise a plurality of emissive elements, each emissive element of a zone being assigned the same characterisation data.
  • the viewing angle under which the matrix display is or is to be viewed at may be selectable by a user or the viewing angle under which the matrix display is or is to be viewed at may be measured using a detection system.
  • the characterisation data may furthermore comprise at least one of dependence on backlight and dependence on an environmental parameter.
  • the environmental parameter may be the intensity of the environmental light.
  • pre-correcting of the drive signal may be performed based on a look-up table. Pre-correcting the drive signal may also be performed at least partly based on using a mathematical function.
  • the method may furthermore comprise generating the characterisation data from images captured from individual zones of emissive elements.
  • Generating the characterisation data may comprise building an emissive element profile map representing characterisation data for each emissive element of the matrix display.
  • the pre-correcting may be carried out in real-time.
  • the pre-correcting also may be carried out off-line.
  • the enforced greyscale display standard may be the Digital Imaging and Communications in Medicine (DICOM) standard published by National Electrical Manufacturers Association.
  • DICOM Digital Imaging and Communications in Medicine
  • the method according to the present invention for correcting non-conformance in greyscale or colour values of at least a plurality of zones of emissive elements in a matrix display may comprise repetitively correcting non-conformance in greyscale or colour values, such that, with a varying correction as a function of time, conformance with the enforced greyscale or colour display standard is obtained and conformance with the enforced greyscale or colour display standard is ensured for changing viewing conditions over time. This correction may be performed automatically.
  • the method also may comprise correcting non-conformance in greyscale or colour values by adjusting the degree of output greyscale or colour depth, i.e. adjusting the number of output greyscale or colour values to allow obtaining or more easily obtaining the enforced greyscale or colour display standard.
  • the invention also relates to a system for correcting non-conformance in greyscale or colour values of at least a plurality of zones of emissive elements in a matrix display, the correcting being with respect to an enforced greyscale display standard.
  • the system comprises a memory means for storing characterisation data characterising the non-conformance in greyscale or colour values of the plurality of zones of emissive elements as a function of its drive signals and as a function of a viewing angle under which the zone of emissive elements is or is to be viewed at, and a correction device for pre-correcting, in accordance with the characterisation data, driving signals to the zone of emissive elements to obtain a greyscale or colour level conform an enforced greyscale or colour display standard.
  • the system furthermore may comprise a characterising device for generating characterisation data for at least a number of zones of emissive elements by establishing a relationship between the greyscale or colour levels of each of the zones of emissive elements and the corresponding drive signal for a number of viewing angles and a number of spatial locations in the matrix display.
  • the characterising device may comprise an image capturing device for generating an image of the emissive elements of the matrix display.
  • the correction device may comprise a viewing angle determination device for determining the viewing angle of a user with respect to a display system.
  • the characterising device may comprise a light-output value assigning device for assigning a native greyscale level value to at least a number of zones of emissive elements of the matrix display.
  • the system may be a part of a matrix display for displaying an image.
  • the invention also relates to a matrix display device for displaying an image.
  • the matrix display comprises a plurality of zones of emissive elements, a memory for storing characterisation data for at least a number of zones of emissive elements of the matrix display, the characterisation data representing a relationship between greyscale or colour levels of a zone of emissive elements and its corresponding drive signals, the characterisation data being a function of the spatial location of the zone of emissive elements and a function of the viewing angle, a means for determining the viewing angle of a user with respect to the matrix display and a correction device for pre-correcting, in accordance with the characterisation data, driving signals to the zones of emissive elements so as to obtain a greyscale or colour level conform the enforced greyscale or colour display standard.
  • the invention also relates to a control unit for use with a system for correction of non-conformance in greyscale or colour values of at least a plurality of zones of emissive elements of a matrix display for displaying an image, the correction being with respect to an enforced greyscale or colour display standard.
  • the control unit comprises means for storing characterisation data for at least a number of zones of emissive elements of the matrix display, the characterisation data representing a relationship between greyscale or colour levels of a zone of emissive elements and its corresponding drive signals, the characterisation data being a function of the spatial location of the zone of emissive elements and a function of a viewing angle, means for determining the viewing angle of a user with respect to the matrix display, and means for pre-correcting, in accordance with the characterisation data, driving signals to the zone of emissive elements so as to obtain a greyscale colour level conform the enforced greyscale or colour display standard.
  • the compensation does not necessarily decrease significantly the contrast ratio of the medical displays, contrary to existing techniques that improve luminance uniformity.
  • the compensation does not necessarily decrease significantly peak-luminance or increase dark-level output of the display.
  • the off-axis DICOM conformance can be obtained for a wide variety of viewing situations, i.e. that the DICOM conformance is obtained for different viewing angles.
  • the teachings of the present invention permit the design of improved methods and apparatus for medical imaging.
  • top, bottom, over, under, left, right, height, width, horizontal and vertical, and the like in the description and the claims are used for descriptive purposes only and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.
  • the invention provides a system and method for adjusting a display system according to an enforced standard for displaying greyscales.
  • an enforced standard for displaying greyscales typically, this problem is encountered in medical imaging, although the invention is not limited thereto.
  • a typical standard used for medical imaging is the Digital Imaging and Communications in Medicine (DICOM) standard published by National Electrical Manufacturers Association.
  • the Greyscale standard is discussed in supplement 28 of the DICOM standard, related to "Greyscale Standard Display Function". Nevertheless, the systems and methods of the present invention also allow compliance with other standards for displaying greyscale levels, in other words the invention is not limited to the greyscale standard of DICOM supplement 28.
  • the invention will be described for the greyscale standard of DICOM supplement 28 for a display system.
  • the display system which may be a medical electronic display system, comprises a display device which preferably is a fixed format display such as e.g. a plasma display, a field emission display, a liquid crystal display, an electroluminescent (EL) display, a light emitting diode (LED) display or an organic light emitting diode (OLED) display.
  • a display device which preferably is a fixed format display such as e.g. a plasma display, a field emission display, a liquid crystal display, an electroluminescent (EL) display, a light emitting diode (LED) display or an organic light emitting diode (OLED) display.
  • EL electroluminescent
  • LED light emitting diode
  • OLED organic light emitting diode
  • a first step in the method of adjusting a display system according to the enforced greyscale standard is characterisation of the emission behaviour of the display system as a function of spatial position and viewing-angle.
  • the transfer curve describes the luminance output (cd/m 2 ) as a function of the digital driving level DDL.
  • a number N of measurement positions is chosen. The exact number of measurement positions is not limiting for the present invention and can be selected based on a trade-off between accuracy and required measurement time, and based on the available memory capacity for storing transfer curve related information present in the display device 200. As illustrated in Fig.
  • the measurement points can be related either to parts of the display device 200 comprising a number of pixels, referred to as a zone 202a, 202b, 202c, 202x, 202y, ... , or to all individual pixels 204i, 204j, 204k, 204m, ... of the display device 200, or to individual sub-pixels (not shown in Fig. 6) of the display.
  • the display device 200 could be an LCD-panel having a resolution of 2560x2048 pixels and this display device could be divided in 15x12 zones, the zones being the measurement points, or the 2560x2048 pixels could be taken as measurement points.
  • either the transfer curve of the centre pixel can be used, as shown for zone 202x with centre pixel 204m, the mean native transfer curve of a group of centre pixels can be used or the mean transfer curve of all the pixels in the zone can be used, as illustrated for zone 202y.
  • the mean native transfer curve of a group of centre pixels can be used or the mean transfer curve of all the pixels in the zone can be used, as illustrated for zone 202y.
  • these measurements can be performed by using a single luminance measurement device with a small acceptance angle and measuring sequentially at the different measurement points on the display device.
  • a good acceptance angle typically is around 3°.
  • Some medical standards (such as DIN6868-57) require acceptance angles between 1° and 5°.
  • a typical single luminance measurement device that can be used is e.g. a CA-210 LCD Colour Analyzer constructed by Konica Minolta Photo Imaging USA Inc, a luminance measurement device with a typical acceptance angle of ⁇ 2.5°.
  • Another possibility is to use a camera system that can measure multiple locations on the display at the same time.
  • camera-systems exist that can perform measurements for several viewing-angles by means of one single image (by using several lenses among which a Fourier-lens).
  • the only requirement is that the measurement device can obtain the transfer curve for the display (sub) pixel or zone (all locations) and for different viewing angles. It is to be noted that these transfer curves can be approximations based on incomplete measurements and interpolation.
  • the spatial and off-axis DICOM-conformance of the display is improved. This is not done by making the display more uniform over its complete display area, contrary to prior art methods, when the only object is to improve DICOM-conformance, as making the display more uniform implies amongst others a decrease in contrast and brightness.
  • Contrast is a measure of different brightness in adjacent regions of an image.
  • An aspect of the present invention is that for every individual display zone or for every individual pixel a DICOM-conformant characteristic is obtained thus following a DICOM-conformant display curve, but that the different pixels/zones can each follow different curves.
  • the allowable error margin for fitting to the DICOM standard is described in e.g.
  • Fig. 7a illustrates the approach of improving luminance uniformity to obtain better DICOM conformance, as known from the prior art.
  • Fig. 7a shows the transfer curve 701, 702 of 2 pixels at different locations of the display screen 200 and also the resulting transfer curve 703 after luminance correction.
  • the resulting curve after correction is chosen so that it is DICOM-compliant, but results in a major decrease in contrast ratio.
  • Fig. 7b illustrates what happens according to a method of the present invention: equalisation of the luminance over the display area is not attempted but rather a correction is performed to the transfer curve 701, 702, of each pixel or zone and this in such a way that the resulting transfer curve 704, 705 for each pixel or zone follows a DICOM-compliant curve.
  • a DICOM-conformant curve for a pixel that has a luminance range of 0.5 cd/m 2 to 500 cd/m 2 can be found but also a DICOM-conformant curve for a pixel that has a luminance range of 1 cd/m 2 to 600 cd/m 2 .
  • the present invention can also be combined with the prior art techniques, such that an increased luminance uniformity, although not perfect, is obtained, while the greyscale-standard conformance is significantly improved and at the same time the contrast loss of the display system is limited.
  • the characterisation data that needs to be provided comprises an identification of the pixel in order to retrieve the native transfer curve information or immediately the corrected transfer curve information, the original grey-scale level, i.e. the digital display level, that was provided for the pixel, and the viewing angle from where the pixel is observed.
  • the identification of the pixel can e.g. be a pixel number, a pixel position on the screen, the pixel column and pixel row, or any suitable alternative representation enabling to identify a pixel.
  • the viewing angle may be provided in different ways, such as being selected at the display system, being selected using a remote control, measured automatically.
  • the viewing angle is defined as the angle between the on-axis direction, i.e. the direction perpendicular to the plane of the display, and the direction user - display zone.
  • the viewing angle equals zero degrees for that pixel or zone.
  • the viewing angle typically can be translated into a horizontal viewing angle and a vertical viewing angle.
  • the horizontal viewing angle corresponds with the projection of the viewing angle on a plane determined by the perpendicular direction to the plane of the display and the direction of the width of the display
  • the vertical viewing angle corresponds with the projection of the viewing angle in a plane determined by the perpendicular direction to the plane of the display and the direction of the height of the display.
  • the horizontal viewing angle during practical use of the display will vary between - 70° and +70°, preferably between -60° and +60° and more preferably between -50° and +50°.
  • the vertical viewing angle during practical use of the display will typically vary between -45° and +45°, although positive viewing angles, i.e. viewing angles whereby the display is positioned lower than the viewing means of the user, are more common.
  • the method and system typically will comprise characterisation data at least for viewing angles within these ranges.
  • the term "user" should be interpreted in the widest possible sense and includes not only animals or humans but also optical viewings systems such as cameras, e.g. as mounted on robots.
  • the display may be calibrated during production or installation with respect to this fixed angle of use, such that during operation no additional input is necessary. If the display is used from different locations, i.e. if different viewing angles can be used, the viewing angle needs to be provided to the display to obtain the optimum DICOM conformance.
  • a remote control device allowing to select the current viewing angle to be used for DICOM adjustment.
  • this can be obtained by for instance using a camera or sensor, e.g. a directional infra-red sensor, built into the display housing.
  • a camera or sensor e.g. a directional infra-red sensor, built into the display housing.
  • a camera or sensor e.g. a directional infra-red sensor
  • the mean value of the viewing angle may be provided to the system.
  • the present invention also includes the use of devices to track the location of the user, e.g. to determine not only the angle of view but also the distance of the viewer from the display. For example, radar or ultrasound can be used for these purposes. The exact way the user location and viewing angle is calculated/measured is not limiting for the present invention. Once the viewing angle and preferably the user distance is known for each pixel or zone this information is used to apply correction to that pixel or zone.
  • Compensation for viewing angle dependency can be applied as if it were independent of the spatial location of the pixel/zone on the display system, i.e. all pixels/zones using the same viewing angle dependency correction data, or it can be applied as being dependent of the spatial location of the pixel/zone on the display, i.e. each pixel/zone having its own viewing angle behaviour. If the highest quality is desired, it is preferred to compensate in accordance with location on the display as the display panel has different viewing angle behaviours at different locations on the panel area.
  • Fig. 8a shows a flow chart of a method 300 for displaying an image.
  • a pixel to be imaged is selected.
  • pixel identification information is obtained which is needed to retrieve the necessary characterisation data for the pixel to be imaged.
  • the input value or P-value for the pixel is obtained, i.e.
  • step 308 it is checked whether the viewing angle for the display system is already known. If this is not the case, method 300 proceeds to step 310 wherein the viewing angle for the display system is determined or obtained, e.g. by checking the status of a switch at the display system, by measuring the viewing angle, or by obtaining the viewing angle from a remote control system. In an alternative method, the viewing angle information is pre-stored in the display system based on measurements on a prototype or mathematical calculations. The obtained characterisation data, i.e.
  • step 312 This digital driving level is then used to drive the pixel thus obtaining an accurate greyscale level (step 314).
  • step 316 it is checked whether other pixels need to be imaged. If it is not the last pixel for imaging, a next pixel is selected; if, the last pixel of the image to be represented has been converted, the correction method ends (step 318) as the whole image is displayed.
  • Fig. 8b In an alternative method 350, as illustrated in Fig. 8b, it is assumed that the viewing angle dependency is not independent of the spatial location on the display system such that the two corrections, for greyscale level and for viewing angle, are coupled and need to be performed at the same time. In other words, this method can be used for a general situation where it is assumed that each position on the display can have a different viewing-angle behaviour.
  • the method comprises the same steps as method 300, but the viewing angle information is specified for each pixel.
  • step 320 is performed wherein the viewing angle information for the display system is used to determine the viewing angle information for the pixel selected in step 302 and identified in step 304.
  • This lookup-table takes as input the P-value (m-bit), an identification of the pixel like e.g. the location of the pixel (row & column, number or zone number) and the viewing angle for the pixel.
  • the output is the DDL that gives best performance for that specific situation.
  • Some medical displays are used both in portrait and landscape orientation. This means that the display can be physically rotated 90°. In that case it is of course not necessary to store the viewing angle behaviour for both orientations.
  • the viewing behaviour can be measured for the orientation that is mostly used (portrait) and if the display is changed to landscape orientation then the viewing angle data can be rotated 90° and used.
  • Examples of such functions can be polynomials; a set of coefficients of cosines functions, ....
  • Another possibility is to reference all characterisation and/or correction data relative to a chosen typical data-set. For instance reference can be made relative to the correction/characterisation of the centre of the display. Typically this technique will require less storage area, as in this case the values of the correction coefficients will be smaller thus resulting in less bits needed to store them.
  • a variant to the reference data/characterisation is to delta-encode the characterisation/correction data, i.e. the difference with the previous data, in this case the neighbouring location or viewing angle is used. Also symmetry in the data can be exploited to reduce the storage requirements.
  • the viewing angle behaviour will have rather good point symmetry around the on-axis point.
  • a somewhat more complex solution is to group or classify the characterisation or correction data into a number of reference classes with the intention to significantly reduce the required storage area. It can for instance be envisaged to group pixels or zones that require the same (or approximately the same, within a pre-set limit) spatial compensation. Instead of storing that compensation data then for each pixel or zone, a small reference class can be stored for each pixel or zone and the actual larger compensation data can be stored only once. The same holds for the viewing angle behaviour. Of course this clustering can be done for spatial compensation and/or viewing angle compensation independently or together. For people skilled in the art it will be clear that lots of algorithms exist to group elements in classes, such as vector quantization, neural networks....
  • lookup tables and circuitry based on interpolation circuits or mathematical functions or a combination thereof can be used. It is furthermore to be noted that it is also possible to combine existing lookup-tables used for image enhancement, with the lookup-tables or compensation needed for the present invention.
  • the correction methods and algorithms described in the present invention can be executed both real-time or offline.
  • Fig. 9 a number of different locations to perform a real-time correction in a system 370 is shown.
  • the system 370 comprises a host computer 372 and a display system 390.
  • the host computer 372 can be any conventional computer providing a significant high quality central processing unit CPU 374 and a significant high quality graphical card 376.
  • the graphical card 376 comprises a software component, which typically can be firmware 378 and a hardware component 380.
  • the pixel correction can be done by the CPU 374 of the host computer 372, such as for example by means of the driver code of the graphical card 376 or with a specific application or embedded in a viewing application.
  • pixel correction also can be performed in the graphical card 376 itself, either in a hardware component 380 of the graphical card 376, or in a firmware component 378 of the graphical card 380.
  • the pixel correction also can be performed in the display system 390 itself, either in display hardware 394 or in display firmware 396.
  • a further alternative is to perform the pixel correction on the signal transmitted between the graphical card 376 and the display system 390, i.e. is somewhere during this transmission in the transmission channel 398.
  • a first component of the system 370 e.g. the CPU 374 of the host computer 372
  • a second component of the system 370 e.g. in the display hardware 394.
  • Fig. 10a, Fig. 10b and Fig. 10c give an overview of different embodiments of methods for calibration that can be used according to the present invention.
  • the calibration method 400 does not include viewing angle dependent measurements but the viewing angle can be introduced from e.g. theoretical considerations or it can be assumed that the viewing-angle behaviour is proportional to the viewing-angle behaviour of a reference display system of the same type. In that case the viewing angle dependency can be characterised once and used for all panels of that type.
  • the calibration method 400 for this embodiment involves the following steps.
  • step 402 the calibration procedure is set up. This is typically done during manufacturing of the system, but it also can be performed at the place of use of the display system, e.g. if due to heating, aging or human intervention, such as e.g. adjusting of the backlighting, the characteristics of the system have been changed.
  • step 404 a zone or a pixel is selected for calibration. As described above, the calibration can either be done on zones in which the pixels are grouped or the calibration can be done on individual pixels or even on sub-pixels.
  • the method then proceeds to step 406 wherein a driving voltage, referred to as digital driving level DDL in the DICOM specification, is selected.
  • DDL digital driving level
  • the condition to be fulfilled is that significant accurate information is to be obtained to substantially obtain the details of the native transfer curve.
  • interpolation can be used between measurement results.
  • the selected driving voltage is then used to drive the selected zone or the pixel in step 408. As discussed above, if a zone is driven, this can either be a central pixel of a zone or a number of pixels in the zone, or it can be all pixels in the zone. Other specific pixel selections from the group of pixels forming a zone also can be used, as will be clear for a person skilled in the art.
  • the luminance of the driven zone is measured using a luminance detection system.
  • step 412 The result of this measurement is stored in step 412, after which, in step 414, it is checked if all driving voltages for the selected zone are already used for obtaining the native transfer curve information. in this way, by driving the zone at different driving voltages, measuring the corresponding luminance level and storing the couples (driving voltage, luminance level) the native transfer curve information is obtained and stored. If all needed information about the native transfer curve for the currently selected zone is obtained, method 400 proceeds to step 416, where it is decided if another zone/pixel needs to be measured. If this is the case, the method returns to step 404, for characterising another zone or pixel. Otherwise all spatial information about the native transfer curves for the display system is obtained and method 400 proceeds to step 418.
  • the information of the greyscale level display standard to be enforced is obtained, in the luminance range needed, i.e. depending on the measured luminance values.
  • the corrected transfer curves for the different pixels/zones of the display system are obtained by fitting the results to the greyscale level display standard information to be enforced.
  • the viewing angle information for the display system which may be based on theoretical considerations or on measurements on a prototype display system, is also introduced, thus resulting in corrected transfer curves for the different pixels/zones and for different viewing angles.
  • a more extended method 440 for calibrating as shown in Fig. 10b, additional viewing angle measurements are performed, thus allowing to optimise the enforced greyscale level display standard conformance for viewing angle dependency.
  • Fig. 10b method steps having the same reference signs as in Fig. 10a are as explained above, and are not explained here in detail.
  • step 406 After selection of the driving voltage in step 406, additional steps 424 and 426 are introduced such that for each zone/pixel and for each driving voltage the native transfer curve information can be stored for a number of viewing angles.
  • the number of viewing angles used to obtain significant accurate transfer curve information depends on the display system used.
  • the viewing angles can be divided into zones and interpolation can be used to obtain an approximate transfer curve for all viewing angles. Using interpolation allows to reduce the measurement time.
  • An alternative method 460 for calibrating, as shown in Fig. 10c, allows to measure the viewing angle dependency for one zone/pixel and uses this viewing angle dependency as the general viewing angle dependency.
  • method steps having the same reference signs as in any of Fig. 10a or Fig. 10b are as explained above, and are not explained here in detail.
  • step 428 it is decided whether the viewing angle dependency for the selected driving voltage is known and if not, the method proceeds to step 424 such that the viewing angle dependency is measured for this zone/pixel. Further in the method, if another zone is selected, in decision step 428, the viewing angle dependency will be decided to be known from previous measurements and the viewing angle dependency will not be recorded anymore. The viewing angle dependency measured for the first zone will then be used in step 420 to obtain the appropriate corrected transfer curves for all pixels/zones. This significantly decreases measurement time since the viewing angle measurements do not need to be performed at multiple locations on the display.
  • the system may comprise a detection system for detecting the status of the back-light.
  • a detection system for detecting the status of the back-light This can be e.g. a detector that allows detection of the emission from the screen such that the intensity of the backlighting can be tested and such that the calibration information for conformance with the DICOM standard, or any other grey-level display standard, can be adjusted accordingly.
  • changes of the native transfer curve of the display can be detected., if e.g. a photo-sensor is placed so that it measures on the front-side of the display area, i.e. the viewing side of the display area.
  • the environmental conditions in the room for viewing can be measured by using a detection system somewhere in the room or preferably in the housing of the display so that the amount of environmental light that is present can be measured, as this will alter the viewing conditions and will influence the DICOM-conformance of the display.
  • a detection system somewhere in the room or preferably in the housing of the display so that the amount of environmental light that is present can be measured, as this will alter the viewing conditions and will influence the DICOM-conformance of the display.
  • An example is given for a medical LCD-panel that has all pixels in dark state having a luminance of approximately 0.5 cd/m 2 and ambient light having a luminance between 0.1 cd/m 2 , i.e. a completely dark radiology room for instance for mammography, up to 30 cd/m 2 in a normal office.
  • the calibration information used for adjusting to DICOM-conformance can be adjusted to influences of external factors. Detection at different locations on the display is possible but not always necessary, as the effects may be proportional for all spatial locations at the display and may be proportional for all viewing angles of the display.
  • the present invention can be applied to any situation where the transfer curve of each pixel or zone under all or some viewing angles needs to fulfil certain mathematical relationships.
  • the transfer curve and more particularly the luminance value of each pixel or zone needed to follow a certain mathematical curve as described by "DICOM / NEMA supplement 28 greyscale standard display function”.
  • DICOM / NEMA supplement 28 greyscale standard display function A simple extension to this model can be that for small viewing angles the transfer curve indeed needs to follow that mathematical relationship but for larger viewing angles the transfer curve is changed to a constant function.
  • the spatial and viewing angle correction can also be adapted to generate the lower number of greyscale values. Because of the lower number of output greyscale values it will typically be easier to comply with an enforced display standard. Warning the user or reducing the number of output greyscale values may be e.g. performed when the viewing angle is outside the preferred ranges as described above.
  • the present invention furthermore is not limited to greyscale displays.
  • a reference work for colour imaging is "Colour Vision and Colourimetry, Theory and Applications” by Daniel Malacara.
  • the invention not being limited thereto, the use of a colour display to view greyscale images is described. In that case the input of the display system is a greyscale image, but the display system itself has colour possibilities.
  • a resulting output having the same luminance but a different colour point as described for example - but not limited to - by CIE colour co-ordinates x,y, can be obtained.
  • These additional degrees of freedom can be used to obtain a specific colour behaviour, which is to be obtained in addition to the greyscale standard display function.
  • a first example of such a specific colour behaviour is selecting a constant specific colour point for the greyscale values.
  • the pixels should follow the specific luminance greyscale standard curve, e.g. the DICOM GSDF, and the colour co-ordinates should remain at a specific, user-selected, value when following this greyscale standard curve.
  • Another example of specific colour behaviour is that, together with the greyscale standard to be complied with, a change in colour is obtained. This can be done by e.g. forcing the colour co-ordinates to comply with a specific curve, e.g. forcing the colour co-ordinates such that a linear change between green and red is obtained when following the greyscale standard curve from minimum to maximum. It will be obvious for a person skilled in the art that variants on standards for colour co-ordinates can also be used and that the invention is not limited thereto.
  • the present invention also relates to a method and system whereby for all pixels and viewing angles, or for a limited number of zones or viewing angles, when changing the input greyscale stimulus from minimum to maximum, the output luminance of the display system complies with a greyscale standard to be followed and for all pixels and viewing angles, or for a limited number of zones or viewing angles possibly different from the ones described above, when changing the input greyscale stimulus, the output of the display system, more specifically the colour co-ordinates comply with a specific selected mathematical curve (for instance a constant, a linear curve between two colour points, ). It is to be noted that the mathematical curve does not need to be constant but that it also can be time-dependent or depend on other parameters such as e.g. external measurement data, external factors, ...
  • R,G,B values of the display system to colour co-ordinates such as the CIE x,y co-ordinates
  • colour co-ordinates such as the CIE x,y co-ordinates
  • This can be e.g. done by measuring the colour-co-ordinates of all or a selection of R,G,B values and applying the inverse transformation if a conversion from R,G,B to x,y co-ordinates is needed.
  • Another possibility is to theoretically deduce the colour co-ordinates for all R,G,B display values based on a limited number of measurements, such as the transfer curve of the R,G and B sub pixels and the colour co-ordinates of the fully-on and fully-off state of the R,G and B sub pixels.
  • the invention also can be used in colour critical images.
  • the display input is a colour image, as described for example by R,G,B values in a specific colour profile
  • the display system also allows colour output.
  • the goal is then to improve the conformance of the display output image to the user selected colour profile and this by applying spatial and viewing-angle corrections.
  • a mathematical relationship can be defined that states that the combination of the three transfer curves of all pixels/zones should result in a specific colour profile. This mathematical relationship allows calculating x,y-colour coordinates from the three colour transfer curves together.
  • the input image typically is specified in R,G,B colour co-ordinates in a specific colour profile.
  • the specific colour profile can be user-defined and may easily be converted to standard colour co-ordinates such as e.g. the CIE X,Y,Z-system.
  • the image to be displayed typically is specified in a standard colour co-ordinate system that differs from the native R,G,B output colour profile of the display system.
  • a spatial and viewing-angle correction system can be applied in the same way as described for greyscale curves.
  • the characterisation data that defines the output - as specified in a standard colour co-ordinate system - as a function of the drive signals the spatial location and the viewing angle can be measured or calculated mathematically.
  • the output can be e.g. specified in the CIE X,Y, Z colour co-ordinate system, and the drive signals can be e.g. given in R, G and B values.
  • Yet another example is for displaying images where absolute colour co-ordinates are less important but differences between colours are important.
  • the spatial and off-axis correction are applied such that differences between colours, as expressed e.g. in colour JNDs, are displayed in the same way for all locations on the display and for all viewing angles.
  • the present invention relates not only to a system wherein an optimised conformance to an enforced greyscale or colour display standard may be provided, it also relates to the corresponding method for adjusting images and displaying adjusted images conform an enforced greyscale or colour display standard and it furthermore also relates to the methods described for calibrating a system such that it is conform an enforced greyscale or colour display standard.
  • the correction method to obtain improved enforced display standard behaviour allows correction for the individual greyscale or colour behaviour of each pixel/zone.
  • the obtained transfer curve for each pixel/zone is such that each of those transfer curves fulfils the enforced display standard behaviour.
  • the obtained transfer curves for each pixel/zone do not enforce all pixels/zones to the same minimum and maximum brightness and even for pixels/zones having the same minimum and maximum brightness, the correction curves may differ to obtain an optimum individual enforced display standard behaviour. In the present invention, therefore, no equal transfer curves for each pixel/zone are provided, but the transfer curve for each pixel/zone is optimised individually.
  • a "time-dependent" correction is provided, depending on at least some circumstances in which the display system is used.
  • the applied correction furthermore allows adjusting the degree of output greyscale depth, e.g. by decreasing the output greyscale depth if for certain large viewing angles no compliance is obtained with the enforced display standard.

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TW094111813A TW200540793A (en) 2004-04-15 2005-04-14 Method and device for improving spatial and off-axis display standard conformance
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US11/578,385 US8228348B2 (en) 2004-04-15 2005-04-15 Method and device for improving spatial and off-axis display standard conformance
KR1020067021179A KR101122982B1 (ko) 2004-04-15 2005-04-15 공간 및 비축 디스플레이 표준 부합성을 개선하는 방법 및 장치
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